It’s the idea that an area of space around a star will be at the right temperature for life to exist. Not too hot, not too cold, hence Goldilocks.

It’s a bit like standing around a campfire on a very cold night. Stand too far away and you freeze, stand too close and you catch on fire and burn to death.

It’s the same with planets orbiting stars too, if they’re too far away then water freezes and life can’t emerge, and if they orbit too close the planet is roasting hot and nothing can live.

It gets a bit more complex than this though, but complex in a fun way. Oh and its also got some pretty big implications for the search for extraterrestrial life…

This Goldilocks zone is more usually called a habitable zone (HZ for short). It’s the distance around a star at which a planet can maintain surface liquid water.

Scientists care about liquid water, as all life on Earth needs liquid water to survive (life on Earth is basically bags of water with a few other ingredients thrown in). Scientists’ care about the idea of a HZ as it guides our thinking as to where in our Solar System life could potentially be found, and where it could be found in other solar systems too. And we all care about finding alien life, right?

Earth is in the HZ of our star, obviously, whereas Venus is too close to the Sun, as it’s surface is almost hot enough to glow, and Mars is probably right at the outer edge of the habitable zone, as its surface is too cold for water to remain liquid for long.

A simplified representation of our Sun’s habitable zone

A habitable zone is therefore defined as the region around a star between the distance at which water would evaporate and the distance at which surface water begins to freeze. (Sometimes the outer edge is set at the distance at which carbon dioxide would freeze out of an atmosphere, as CO2 is a greenhouse gas that can heat a planet, meaning that planets rich in CO2 could be warm enough to enjoy liquid water at a distance a bit further out than we would normally expect to find it).

The HZ doesn’t just depend on the distance from a star though; it also depends upon the features of the planet. If Mars had been slightly bigger it would have been able to maintain an atmosphere (it lost most of it’s initial atmosphere to space as its gravity isn’t strong enough to capture it permanently, more here) and if this atmosphere contained enough greenhouse gassed Mars could have a warm and wet surface today. Thus a HZ is typically defined as the region around a star in which an Earth-like planet could maintain surface liquid water.

A HZ also depends upon the star too. Larger stars emit much more heat, thus the zone in which an Earth-like planet could maintain surface liquid water would be much further out than for our Sun, and much closer in for stars smaller than ours.

Like this (click to enlarge)

Habitable zones are also affected by time. Over their lifetimes the heat output of stars changes. Our Sun has increased in luminosity since it first formed and is roughly 30% hotter today than it was 4.6 billion years ago. This means that the habitable zone must have moved outwards throughout the life of our star. Astronomers and astrobiologists believe that Earth has always been inside our Sun’s habitable zone, but it inspired a scientist called Michael Hart to come up with the idea of the Continuously Habitable Zone (CHZ). This is the region around a star in which an Earth-like planet can sustain surface liquid water for most of the lifetime of its star.

The idea of a CHZ is important, as the fossil record indicates that it took a long time for complex life to evolve on Earth. Palaeontologists have discovered that single-celled life emerged early in Earth’s history, possibly as far back as 4 billion years ago, but that it took more than 3.5 billion years for this bacterial life to evolve into the first animals. If the Earth had formed 5% closer to the Sun, or 15% further away, its likely that it would have been outside of this CHZ and thus animal life would not have been able to evolve on Earth (yes, that includes us).

This leads to a really cool habitable zone idea, that there may be different HZs for different types of life, an Animal Habitable Zone (AHZ) and a Microbial Habitable Zone (MHZ).

It’s likely that the AHZ would be very narrow, and would be confined to a star’s CHZ, as the planet would need to have surface liquid water for billions of years to allow animals time to evolve.

The animal habitable zone, narrow

The MHZ will likely be much wider, as microbial life may well take a mere few hundred millions years or so to emerge, thus can live on planets that may only spend a short time in a star’s HZ. Venus and Mars may well have had their own microbial life early in their histories, and thus may have been inside our Sun’s MHZ for a time.

Two other discoveries have also expanded the possible boundaries of a MHZ. The first of these was the discovery of extremophiles in the 1970s. Extremophiles are single-celled life forms that thrive in extreme conditions such as boiling water, sulphuric acid or inside rocks deep within the Earth’s crust. Extremophiles expanded the range of conditions in which life can be found and thus expand the range of the MHZ.

The second discovery is that liquid water can exist below the surface of planetary bodies that orbit way outside of a star’s HZ. Evidence suggests that some of the moons orbiting gas giant planets in our Solar System, such as Europa and Enceladus, may have vast subsurface oceans that could support life. I won’t go into the details here (if you want to know more than please see these posts; Europa, Enceladus) but it’s possible that these moons may have their own biospheres in underground oceans, but its more likely that these biospheres are microbial rather than animal. The existence of these moons suggests that the MHZ may be huge, and could potentially span between the orbits of Venus and Saturn in our Solar System.

Europa, within our Sun’s microbial habitable zone?

So what does this mean for the search for alien life?

Firstly, it means that if we hunt for advanced alien life, such as alien civilisations with radio technology, then we need to confine our searches to exoplanets that orbit in a very narrow CHZ around their stars.

Secondly, it suggests that microbial life may be relatively common in our Galaxy, as the MHZ is potentially so wide, but that complex life may be extremely rare, as it likely requires a planet of the right size and composition to orbit stars with a stable temperature at a precise distance. This means that planets that can support animal life in our Galaxy may be rare.

So maybe we aren’t alone in our Galaxy, but maybe most of our alien cousins are simple bacteria.

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5 Responses to “Goldilocks, and other Habitable Zones for Life”

Mmm. Thanks for this comprehensive explanation of habitable zones. I learned some new things I hadn’t considered, like the idea of planet-specific habitable zones.

I’m curious as to why you say Europa is more likely to support microbes than animal life. I know it’s not a photosynthetic world, but my impression had been that there’s a lot of water and probably a lot of heat energy under Europa’s ice. I don’t know enough biology to know if there’s a reason that may not be as hospitable to life as an Earthlike photosynthetic environment. I’d love to know your thoughts on the matter.

Its a great question. Generally the consensus is that microbial life needs liquid water, some organic molecules and an energy source in order to emerge and prosper. And hopefully Europa will satisfy these conditions and at least have simple life (fingers crossed!)

But there is a lot of debate about what conditions you need for more complex life to evolve. Life on Earth emerged roughly 3.8 billion years ago, but stayed single-celled for probably around 3 billion years, and then relatively suddenly multicellular life emerged and diversified into animals, plants and fungi.

No one knows why life on Earth stayed so simple for so long. Perhaps life requires a specific set of conditions in order for more complex organisms to evolve and prosper? As Europa is very different from Earth, the prudent thinking is that Europa may not have these conditions and will probably not have complex life. Maybe complex life can only form on terrestrial planets, with large stabilising moons, plate tectonics and oxygen-rich atmospheres?

This isn’t definite though, and I hope the liquid water below Europa’s surface is rich with complex fish analogues and intelligent many armed mollusc-like creatures 🙂

That is a good point I hadn’t thought of. I always assumed multicellular life just took time for the correct mutation to occur–I never thought that it could need certain conditions.

I feel like it’s still such an open field. I’ve heard a lot of talk about how, say, we can figure out how amino acids could be naturally created–but we can’t figure out how peptide bonds between them could come about. It seems we truly don’t know if life is so staggeringly common that it exists on most planets with water, or if it is so staggeringly rare that Earth had to be seeded with cells from space!

I’m very intrigued by the thought of what we’ll find on both Europa and Mars in the coming years–as well as possibly Titan! I was just talking with someone about the funding dilemma–should we be using our resources to fund space exploration, or to solve problems here on Earth? I’m of the opinion that if every country could scrap its Department of War, we could get so much more done on both fronts…our “Defense” Department gets more funding for satellites annually than NASA does, and it consumes many times the total cost of a manned mission to Mars every year! I suppose we can try to figure out how to make that happen…

Yes, lets work together towards a common goal an leave strifte, war and conflicts behind. We have set aside evolution and have to take responsibility as a single thinking entity. The only way to procur saftey as a species is to travle to the stars. Otherwise the next stray nuclear catastrophe, asteroid impact, renegade AI, genetic mutation or slow decline of birthrate will be the end of it. I’m really sick and tired of the slow progress of things. Space need to be explored, and that is now.

Drakes equation takes into account the time a techological civilization will last. So far we have reached 0,00001% of the time of life and the planet and already signs equal to the worst holocausts of all time shows…

I fear that once depravation set in (global heating, peak oil, stagnation et.c.) no funds will be comming to towards space exploration – the only long lasting solution, really. So time is short.

Really if it took 3 bilion years to go from single cell to multicellular the chance of that happening again somwhere else is staggering small. We have to face the possiblilty that we as a thinking organism is a rare occurence and that even if another thinking form of life exist somewhere, the distance to travle to them takes longer than the life expectancy of any of us.

The Sun’s life zone is hard to calculate due to current conditions on Venus and Mars. Mars is of course, too small and is geogically dead, and Venus never shed its dense carbon dioxide atmosphere, making it way too hot. The most I have right now is a school book showing Venus and Mars within, but close to the edges of the zone. They were only half right in saying Venus was too close to the edge. We DO know though, if Venus had the rotation and life and atmosphere of Earth, water could exist as a warm liquid, there would be clouds also, and the feedback loop would mitigate some of the effects of the closer proximity to the Sun. Most of us also know that if we were to use all the various planetary masses like from .10 to 4.00 the mass of Earth at Venus’ distance, there is no possible ways all your worlds would come out exactly the same.(It is certain you would not get runaway greenhouse 100% of the time, but of course, this is ALWAYS assuming an oxygen, nitrogen atmosphere at 1 bar atmosphere pressure) For the smallish worlds, like between .18 to .30 the mass of the Earth, the closer proximity to the Sun could help keep the planet tectonically active for longer, though you also need a magnetic field. Even Earth could have lost all of its water if conditions had been just a little different, even at the current location from the Sun.The most likey results would be the small world like .10 the mass of Earth dying, and losing almost all its atmosphere, the slightly larger worlds might get to retain warm liquid, and a mix of tropical Earths, and Venusian ovens, depending on atmospheric composition, and planetary composition. You may get a super hot super Earth, and a few warm but hot ones still able to support life, again using clouds as a model which reflect some sun.